5-HIAA is a tryptophan metabolite that is excreted in large quantities in the urine of individuals with malignant carcinoid tumours. Sjoederson, A., "Clinical And Laboratory Features Of Malignant Carcinoid", Arch. Intern. Med. 102, 136 (1958). 5-HlAA is used as a diagnostic marker for the disease. Normal values should not exceed 6 mg of 5-HIAA excreted in 24 hours. Goldenberg, H., "Specific Photometric Determination Of 5-Hydroxyindoleacetic Acid In Urine", Clin. Chem., 19, 38 (1973).
Methods for determining levels of HIAA are known. The most widely used method is that of Undenfriend et al., "The Identification Of 5-Hydroxy, 3-Indoleacetic Acid In Normal Urine And A Method for Its Assay", J. Biol. Chem., 216, 499-505 (1955), as modified by Goldenberg, supra. Goldenberg's modification of the method of Undenfriend et al. is based on oxidative coupling of 5-HIAA to .alpha.-nitroso, .beta.-napthol in a reaction catalyzed by nitrous acid to yield a purple complex which can be monitored spectrophotometrically at 540 nm. Because .alpha.-nitroso, .beta.-napthol also absorbs at the same wavelength as the detected species 540 nm it is necessary to remove unreacted .alpha.-nitroso, .beta.-napthol prior to measurement. The latter step may result in some loss of the detected species, as well as necessitating additional steps in the procedure.
Accordingly, it is desirable to develop a simplified spectrophotometric assay for 5-HIAA which avoids the necessity of separating the detected species from other reactants used in the assay prior to spectrophotometric determination. That improvement will facilitate development of automated test equipment for determining 5-HIAA.